In a predatory world, sleep doesn’t make much evolutionary sense. Why would any creature lie down, shut its eyes and not move for about a third of a day? It’s like an invitation to be eaten.

Now a new study out of the University of Rochester Medical Center (URMC) suggests sleep is necessary for the brain to get rid of waste. The study, published in the journal Science, also reveals that the brain’s cells shrivel up by 60% while we sleep to wash away the cellular garbage more effectively. Researchers believe that all the cleaning activity is one of the reasons the brain uses about as much energy while in sleep as while in wake mode. Sleep is known to perform many critical functions, including consolidating memories and recharging cells, but this is the first time it has been linked with cerebral rubbish removal.

The new findings are a continuation of work that Dr. Maiken Nedergaard, the lead author of the current study, published in 2012 in the journal Science Translational Medicine. In that report, she and her colleagues used a new imaging technique, two-photon microscopy, to show that mice brains, which are good models of human brains, have their own plumbing system for disposing of molecular detritus. Known as the glymphatic system, it hitches a ride alongside the brain’s blood vessels to pump cerebrospinal fluid through the brain tissue and wash waste back into the circulatory system, where the liver eventually disposes it.

The new study, also in mice, suggests that while the body powers down, the glymphatic system ramps up, becoming 10 times more active than when the brain is awake. And because the brain’s cells shrink during sleep, the researchers believe, it’s easier for the fluid to flush out its many corridors. During sleep, proteins that are responsible for Alzheimer’s disease and other neurological disorders are also removed more efficiently from the brain than during waking hours. The body’s chief sanitation worker, the lymphatic system, isn’t sufficient for cleaning out the brain because of the brain-blood barrier, the body’s equivalent of a picket line, which prevents certain cleansing agents from passing through the blood and into the brain. But the glymphatic system, composed of cerebrospinal fluid, is given a special pass into the brain, and can pull out toxins and other waste products, primarily during sleep.

The findings raise interesting questions about how sleep may affect the progression of Alzheimer’s or other neurogenerative disorders, but they also provide a strong warning for anybody who skips sleep. The short version: don’t.

“The brain only has limited energy at its disposal and it appears that it must choose between two different functional states – awake and aware or asleep and cleaning up,” said Nedergaard, a co-director of the URMC Center for Translational Neuromedicine, in a statement. “You can think of it like having a house party. You can either entertain the guests or clean up the house, but you can’t really do both at the same time.” And you can’t do either one without the other for long.